This invention relates to a device for measuring reverberation characteristics in a transmission system by a digital squaring and integrating computation process.
Known in the art of an acoustic measurement in a chamber, particularly measurement of transient characteristics such as reverberation time, is the impulse response squaring and integrating process devised by M. R. Schroeder. The basic principle of this process is to measure, when the source band noise is stopped from a steady state, transient characteristics of a received signal S(t) at a sound receiving point. That is, inherent transient response is expressed by an ensemble average of the squared noise decay &lt;S.sup.2 (t)&gt;, which is expressed on the basis of impulse response r(x) between the sound source and the sound receiving point, by the following equation ##EQU1## where N is power of the source band noise.
Accordingly, by squaring and integrating the impulse response r(x) in the integration interval [t, +.infin.], the ensemble average of the squared noise decay &lt;S.sup.2 (t)&gt; at the time point t can be obtained.
FIG. 1 schematically shows the prior art reverberation measuring device implementing the above described process. In a measurement room 1 where reverberation characteristics are to be measured, there are provided an impulse source (e.g. a speaker) 2 which generates a test pulse and a microphone 3 which collects the generated test pulse signal. The signal h(x) collected by the microphone 3 is amplified by an amplifier 8 and is converted to digital data by an analog-to-digital converter 4 and thereafter is squared and integrated by a squaring and integrating circuit 5. The output data of the circuit 5 is applied to a reverberation characteristics computing circuit 6 in which the data is sampled and sequentially loaded in a random-access memory 6a. A calculator 6b calculates reverberation characteristics such as reverberation time on the basis of the data loaded in the memory 6a. The result of the calculation is displayed in a display unit 7a.
If the analog-to-digital converter 4 has a word length of e.g. 12 bits and the sampling frequency is set of 50 kHz in the prior art reverberation measuring device, in order to measure reverberation time up to 60 second the squaring and integrating circuit 5 must have a word length of 40 bits or more. In the prior art device, the word length required for computation of reverberation characteristics is the same as that used by the squaring and integrating circuit 5. For this reason, a large scale computing circuit 6 is necessitated and, moreover, a long calculation time is required.
As regards displaying of the level of the input signal in the prior art device, the level of the input signal h(x) through the amplifier 8 and a signal level detection circuit 9a is displayed by a display unit 7b. Alternatively, the output of the analog-to-digital converter 4 is applied to a signal level detection circuit 9b and the detected level is displayed by a display unit 7c.
In either case, the circuit portion for detecting and displaying the input signal level (i.e., the signal level detection circuit 9a or 9b and the display unit 7b or 7c) is provided separately from the circuit portion for computing and displaying reverberation characteristics (i.e., the squaring and integrating circuit 5, the reverberation characteristics computing circuit 6 and the display unit 7a). This necessitates a complicated circuit design with a resulting large scale circuit.